In many different communications applications, such as mobile phones or any other transceiver, a common signal path is coupled both to the input of a receiver and to the output of a transmitter. In such a transceiver, an antenna may be coupled to the input of the receiver and to the output of the transmitter. A duplexer is, thus, used to couple the common signal path to the input of the receiver and to the output of the transmitter. The duplexer provides the necessary coupling while preventing the modulated transmit signal generated by the transmitter from being coupled from the antenna back to the input of the receiver and overloading the receiver. A duplexer generally consists, thus, of three ports. At the antenna both signals coexist representing incoming and outgoing signals. To avoid collision of these signals, usually different frequency bands are allocated to the Tx and Rx path. The main purpose of a duplexer is to direct incoming Rx signals to the receiver port and to guard TX signals from the TX port to the antenna. To this end, a duplexer usually comprises two highly selective radio frequency (RF) filters for the respective Tx and Rx frequency bands. The RF-filters need to have sufficient rejection for the opposite band. At the antenna port, both filters are connected in parallel. In order to avoid that the two filters degrade each other's impedance a matching network is required.
FIGS. 1 and 2 show exemplary embodiments of such duplexers according to the prior art. In FIG. 1, the antenna port is coupled directly to the transmitting filter 110 (Tx) whereas a transmission line 130 is used to couple the antenna port with the receiving filter 120 (Rx). The transmission line is usually a 90° phase shifting device for transforming the impedance of the Rx filter 120. The Tx filter usually comprises a filter 111 with acoustic resonator devices such as thin film resonators or as applied in the present invention bulk acoustic wave resonators (BAW). An optional shunt matching network. 113 and optional output matching network 112 are provided depending on the respective application as shown in filter 110 of FIG. 1. The Rx filter 120 comprises similar elements with BAW filter 121, and networks 122 and 123.
FIG. 2 shows another exemplary embodiment of a duplexer 200 according to the prior art. Similar elements carry similar numerals. In this embodiment a so-called pi-network 210 is used instead of the transmission line 130. The pi-network 210 consists of two capacitors each connected on one side with ground and coupled on the other side through an inductor. Both implementations, the transmission line 130 as well as the pi-network 210 provide for a +90° phase shift and impedance transformation. Both function as a delay line, the transmission line 130 comprises a quarter wavelength and the pi-network comprises shunt capacitors and a series inductance. The +90° phase shifting is required to transform the input impedance which the Rx filter presents towards the antenna to high values at the Tx frequency range to effectively block the TX signals. The input impedance of the Tx filter in the Rx frequency range is typically high by default and does not require additional impedance transformation or phase shifting.